1. FIELD OF THE INVENTION
This invention relates to ion beam lithography and, more specifically, to a technique for alignment of a mask and wafer for ion beam lithography.
2. BRIEF DESCRIPTION OF THE PRIOR ART
In order to fabricate integrated circuits with increased component density, it is desirable that lithographic lines be formed which are as narrow as possible. Until recently, about a one micron photolithographic line was the minimum width available. Recently, using optical techniques, photolithographic lines having widths as thin as 0.3 microns have been reported. However, it is desirable to obtain lithographic lines of even smaller dimension in the sub 0.1 micron range.
Masked ion beam lithography is a technique by which extremely small features can be patterned. Masked ion beam lithography has the advantage of higher resolution and of speed and economics as compared with x-ray lithography.
In masked ion beam lithography, a mask consisting of transmissive and opaque regions corresponding to an integrated circuit pattern is illuminated by a flood beam of 50 to 200 keV ions. The ions passing through the mask deposit their energy in the resist in a very efficient and spatially confined manner. The deposited energy creates a latent image of the mask pattern in the resist which can be developed. There is little lateral scattering or longitudinal straggle since ions interact primarily with the lighter resist electrons. Consequently, the latent image in exposed resist is sharply defined, both laterally and vertically. The resolution limit is determined by the range of secondary electrons (less than 10 nm) and the contrast of the mask. This intrinsically high contrast leads to wide exposure latitude and a relaxation of resist requirements. Ions expose resist very efficiently. Critical doses are typically one order of magnitude smaller than for electrons and two orders of magnitude smaller than for x-rays. Features smaller than 30 nm have already been printed using the technique of the subject invention. Ion sources of high brightness are commercially available at reasonable prices.
Masked ion beam lithography has been implemented both in 1:1 proximity printing and demagnifying ion projection lithography, where a stencil mask is illuminated by low energy ions (5 keV) which are accelerated and demagnified by an electrostatic optical system. X-ray lithography, also a proximity printing process, is less efficient, more costly and has poorer resolution than ion beam lithography.
A remaining problem in ion beam lithography has been the inability to align multiple layers with each other with an accuracy comparable to the obtainable resolution. The prior art has used principally optical techniques to align x-ray masks or ion beam masks to wafers. Typically, a diffraction grating has been placed upon the mask and the wafer and a light beam, such as from a laser, has been directed to the gratings with optical techniques then being used to align the marks on the wafer with the marks on the mask. Such techniques are complex and still do not provide the degree of alignment accuracy now required. These techniques are also not suitable for ion projection lithography, where the mask and wafer are too far apart to make them practical. In ion projection lithography, detection of the secondary electrons emitted from grooves in the wafer when the ion "image" of slots in the mask impinge on them has been used for alignment. The disadvantage of this technique is that the secondary electrons have a very short range in resist, so essentially all resist must be removed from the grooves on the wafer before alignment can be performed. The technique in accordance with the present invention can tolerate much thicker resist coatings on the wafer.
It is also known that when an ion strikes silicon dioxide, the silicon dioxide emits deep blue light. It has been experimentally determined that the yield is greater than 0.5 photons per proton at 100 keV ion energy. Other materials, such a silicon or polymethylmethacrylate (PMMA), a typical proton resist, do not emit observable amounts of light. This has suggested an alignment system based upon the detection of this light in accordance with the present invention.